Charge pump power supply with noise control

ABSTRACT

An embedded power supply for providing a voltage on a detector module within an imaging system provides the required potential to the module from charge stored on an output capacitor. Charge on the capacitor is replenished by injecting, commonly referred to as pumping, current into the capacitor by pulses of current generated by switching mode circuitry. Charge pumping into the capacitor is efficient because energy is stored in low-loss passive components and transferred into the low-loss output capacitor through low-impedance paths. Switching noise of the power supply is eliminated by turning off the charge pumping circuit during periods when such noise would disrupt the operation of the module, for example when the module is reading out image data. The output capacitor is large enough to supply the required voltage to the module for a certain period when the capacitor is not being pumped.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. application Ser. No.10/958,689, filed Oct. 4, 2004, which claims benefit of the priority ofU.S. provisional application No. 60/508,432, filed Oct. 2, 2003, thedisclosures of which are herewith incorporated by reference.

BACKGROUND

An exemplary imaging system may be composed of four basic subsystems:(1) one or a plurality of detector modules 100 that capture the rawsignals and may perform some signal processing, (2) an image processingsubsystem 110 that converts the information from the detector modules100 into a form suitable for further processing in a computer, (3) aprocessing part 120 that receives the data from the signal processingsubsystem 110, then generates and may enhance one or a plurality offinal images and may also calculate the values of attributes of theobject imaged, and (4) one or a plurality of output devices 130 such ascomputer monitors and printers that present the final images and otherdata in desired formats.

Some imaging modules may require a power supply voltage that isdifferent than the input voltage. This different voltage may begenerated on the module itself. There may be many reasons to generatethe voltage within the module, including reduction of the number ofpower supply lines, reduction of coupling of noise into the system bythe secondary power lines, as well as reduction of fixed pattern noisecaused by differing voltage drops on different lines.

The production of voltages on the chip, however, may itself createnoise. That noise may degrade the received image.

SUMMARY

The present disclosure describes a power supply for providing asecondary voltage on an image detector module using a specializedswitching power supply with reduced noise effect on the final image.This is done according to the present system by using a “switching” typepower supply that operates only during times when specifiedimage-acquiring processes are not being carried out. The power supplyoperates during times when the system is less sensitive to the noisebeing produced. One embodiment of such a switching power supply uses acharge pump that stores the created voltage/charge into a capacitor,that is sized to maintain the power during times when the power supplyis not operating.

DESCRIPTION OF DRAWINGS

These and other features and advantages of the invention will becomemore apparent upon reading the following detailed description and uponreference to the accompanying drawings.

FIG. 1 shows a block diagram of an imaging system comprising foursubsystems.

FIG. 2 presents a schematic diagram of an embodiment of a power supplyused on a chip, according to this system.

DETAILED DESCRIPTION

An embodiment operates by generating secondary voltages directly on adetector module 100 that can operate free of switching noise for acertain period while the module 100 is performing noise-sensitiveoperations such as acquiring detector data.

The embodiment describes use with gamma ray imaging modules. However,this technique may be used in other applications, and specifically inany application where images are acquired.

Gamma ray imaging modules 100 commonly employ p-i-n photodiodes fordirect detection of gamma rays or for indirect detection, in whichphotodiodes detect light emitted from scintillators in response toillumination by gamma rays. Such photodiodes typically operate underreverse bias voltages ranging up to 100 volts or more. The current drainof a module 100 on the reverse bias line is small—typically a fractionof one microampere, and relatively insensitive to the value of thereverse bias.

The signals detected by the photodiodes in a gamma ray imaging module100 are very small—of rough order of magnitude one femtocoulomb pergamma ray detected. Accordingly, the gamma ray module 100 should detectsignals under conditions of the minimum possible electromagnetic noise.Therefore any power supply operating on the module must be as noise-freeas possible while the image data is being acquired.

The embodiment is shown in FIG. 2. The power supply 200 is a switchedpower supply. Power is produced by switching the supply of charge to acharge storage part between on and off. In the embodiment of FIG. 2, thepower is stored on an output capacitor 250. Charge on the capacitor 250is replenished by injecting current into the capacitor using pulses ofcurrent, generated by switching mode circuitry, here shown as FET 230,and inductor 220.

Switching noise of the power supply is reduced or eliminated duringperiods when such noise would disrupt the operation of the module. Thisreduction is carried out by turning off the switching circuit duringthese times. This reduces or eliminates the noise produced by thatswitching circuit. This imaging device may acquire images for specifiedportions of the operation time, and/or may process those images duringthat time.

In this embodiment, the output capacitor is selected to be large enoughto supply the required voltage to the module for a certain period whenthe capacitor is not being pumped. The power supply may be off for 10times as long as it is on, or more. Hence the capacitor may be larger inpower storage capability then a comparable capacitor in other kinds ofcomparable power supplies.

More generally, the power supply of FIG. 2 is a charge pump type ofswitching power supply, and has two basic sections: output capacitor 250and a charge pumping circuit. Output voltage and current are suppliedfrom charge stored on output capacitor 250. Charge on output capacitor250 is replenished by the charge pumping circuit comprised of inductor220, normally-off transistor 230 and Zener diode 240.

The lines labeled VDDP and VSSP supply primary power to the circuit.VSSP is usually called a reference potential. The magnitudes andpolarities of all voltages are taken with respect to VSSP.

A charge pump power supply may provide a voltage of approximately 50 to60 volts at the terminal labeled VDET. This terminal supplies thereverse bias for the photodiodes on the gamma ray imaging module 100.

In the implementation shown, VDDP and the output voltage VDET aredescribed as being positive with respect to VDDP. The same techniquescan, however, be applied to implementations in which the polarity ofVDDP is reversed or the polarity of VDET is reversed or the polaritiesof both VDDP and VDET are reversed, as would be understood by thosehaving ordinary skill in the art.

The charge pumping circuit maintains charge on output capacitor 250. Acontroller 260 produces pumping pulses to the base of the transistor230. The controller can simply be formed of discrete components, such ascapacitors and inductors, or may be more complex, such as amicroprocessor.

During pumped operation of the circuit implementation shown,positive-going pulses are applied to the gate of normally off transistor230 via the line labeled VDETPUMP, turning the transistor on and causingcurrent to flow from VDDP to VSSP through inductor 220 and transistor230. At the end of each pulse, transistor 230 turns off, interruptingcurrent flow through inductor 220. In response to this interruption, theinductor produces a voltage spike on the circuit node labeled VINDUCTOR.This spike is positive with respect to VDDP. During the period when thepotential VINDUCTOR is more positive than VDET, Zener diode 240 conductscurrent in the forward direction from the node labeled VINDUCTOR intocapacitor 250, increasing the charge stored on capacitor 250 and themagnitude of VDET. When the potential VINDUCTOR is no longer morepositive than VDET, the Zener diode stops conducting in the forwarddirection.

Following the end of the voltage spike on the node labeled VINDUCTOR, ifVDET is more positive than VDDP added to the reverse breakdown voltageof the Zener diode 240, 51 volts in the embodiment shown, the Zenerdiode will conduct current in the reverse direction through inductor 220from VDET to VSSP. Therefore, this component has the effect of limitingthe maximum value of potential VDET to the sum of VDDP and the reversebreakdown voltage of Zener diode 220. Other forms of voltage regulationmay alternatively be used.

The power supply of this embodiment is efficient. In the embodimentshown, except for the brief period when transistor 230 is switchingbetween its off and on states, current flows through one or morelow-loss components such as inductor 220 and its small parasiticresistance, transistor 230 and its small on-state resistance, Zenerdiode 240 and its low forward series resistance and low reverse seriesresistance when the reverse bias exceeds the breakdown voltage, alsoreferred to as the Zener voltage, and capacitor 250, which has a highshunt resistance.

The power supply described generates electromagnetic switching noiseduring intervals of charge pumping during either or both of twoconditions: (a) when the current in inductor 220 is changing and/or (b)when capacitor 250 is being charged. This noise could adversely affectdata collected or being processed during these intervals, especiallywhen the data are low-level signals such as the photocurrent of thephotodiodes and the signals in the first level of amplification anddiscrimination of these currents.

The power supply is operated with no switching noise during periods ofimage acquisition, e.g., image signal collection and image signalprocessing by using the controller 260 to stop the charge pumpingprocess during these intervals. During the stop time, the line VDETPUMPis held low, thereby maintaining transistor 230 in the off state. Duringthese quiescent periods, capacitor 250 maintains its stored charge, andmaintains the voltage VDET so long as the stored charge lasts. Biascurrent is supplied to the photodiodes from the charge in the capacitor.However, VDET will decrease steadily during these periods. The size ofcapacitor 250 is preferably selected to limit the extent of the decreaseof VDET during quiescent periods to an acceptable value, e.g., toprevent the voltage VDET from reducing by more than 5% or 10% or someother number. Further filtering can be added at the output VDET tofurther minimize switching transients on this node during the pumpingphase. For example, small value capacitors (e.g., 0.1 uF or 0.01 uF, orboth) may be used as decoupling capacitors.

Power supplies in other embodiments may employ other circuit layouts.For example, a plurality of switched capacitors may be used to pumpcharge onto an output capacitor, rather than employing a switchedinductor. As in the embodiment shown, pumping circuits employingswitched capacitors can be efficient, because energy is stored on lowloss passive elements and flows through low impedance paths.

The component values and the frequency and duration of the pulses ofVDETPUMP may be selected so that the period of time required for chargepumping is small (e.g., 1/10, or less) compared to the required lengthof the quiescent periods.

Invalid data collected or processed during periods of charge pumping canbe purged by filtering data generated during this period. Such filteringmay be performed in software, hardware or firmware, singly or in anycombination. Determining when to disable the function of charge pumpingor when to filter invalid data can be accomplished, for example, bynon-programmable or programmable circuitry (such as a microcontroller).The same controller may also perform the functions of controller 260.

In a detector module 100 with an embodiment of a power supply, theVDETPUMP signal may be generated on the module 100 and one or both ofTpump, the duration of the pumping phase, and Tquiet, the duration ofthe quiet or non-pumping phase, may be determined by values stored inon-chip memory. Alternatively, one or both of these times could bedetermined by a synchronization signal generated off-chip. Use of suchan external synchronization signal may be advantageous in systems with aplurality of detector modules 100 by permitting the system to controlthe readout interval for each module 100.

This circuit has been reduced to practice in a first embodiment within agamma-ray detector module 100, approximately 2.5 cm wide, 5.0 cm longand 5.9 cm high (1 in×2 in×1.5 in). The module 100 includes two main PCBassemblies—a multichip module, or MCM, and a power/interface board, orPIB. The MCM comprises a plurality of segmented photodiode arrays, eachsegment forming a pixel which is associated with a correspondingsegment, also forming a pixel, in a segmented scintillator array, aplurality of application specific integrated circuits, or ASICs, and aplurality of other minor components. When a signal gamma ray emittedfrom the patient or other subject being imaged is absorbed in ascintillator pixel, the scintillator material emits a flood oflow-energy photons, typically in the near-UV to visible range ofwavelengths. The corresponding photodiode pixel absorbs these photons,generating a photocurrent pulse that is injected into a correspondinginput channel in a readout ASIC that determines if the total charge inthe photocurrent pulse is within a range characteristic of a validsignal gamma ray. When such a valid photocurrent pulse is detected, thereadout ASIC and one or more other ASICs on the MCM process the currentpulse information and generate a plurality of output signalsrepresenting quantities such as the energy of the gamma ray detected andthe position of the scintillator pixel that detected the gamma ray.Other embodiments may generate output signals for other quantities, suchas a time associated with the detection of a gamma ray.

The PIB, connected to the MCM, comprises a charge pump power supply, orCPPS, according to this system, and an input/output interface systemthat receives power from external power supplies, distributes theexternal supply voltages and the internal supply voltage generated bythe CPPS to the appropriate terminals of the MCM, and manages the inputof control signals and output of data signals from the appropriateterminals on the MCM.

In this first embodiment, the CPPS was configured as shown in thecircuit of FIG. 2, with approximate values of VDDP=3.5V, VSSP→OV (?),L=100 mH (the value of inductor 220), C=1 mF (the value of capacitor250), Vz=51V (the reverse breakdown voltage or “Zener” voltage of Zenerdiode 240, type BZX84C51, and n-channel FET 230, type BSS123.

The VDETPUMP signal is generated by a microprocessor on the PIB, whichforms the controller 260, as well as carrying out other image relatedfunctions. Pulse parameters such as number of pulses, width of pulsesand pulse frequency were determined by values stored in themicroprocessor.

During charge pumping, the VDETPUMP waveform was a train of 50positive-going pulses of approximately 3.5V amplitude relative to VSSP,a pulse width of roughly 5 ms, a pulse rate of roughly 100 kHz. Thetotal duration, Tpump, of the pulse train was roughly 500 ms. Duringsignal acquisition, VDETPUMP was held near 0V for a quiescent period,Tquiet, of approximately 10 s. More generally, however, the pumpingtime, or time that the power supply is being charged, is preferably atleast 10 times less than the time that the power supply is off.

During the quiescent period, VDET, the output of the CPPS, decreased byroughly 100 mV from its peak value of approximately 54V.

The module 100 of this first embodiment of the invention was testedunder irradiation with 57Co (cobalt-57) gamma rays of approximately 122keV photon energy. During testing, we added a series resistor betweenthe VDET terminal of the CPPS and the power input terminal of the MCM toprovide additional output filtering to minimize the size of thetransients on the MCM generated during pumping. Including these filtercomponents, the total component count of the CPPS, including theVDETPUMP generator, is 6. There was no shielding between the CPPS andthe MCM other than that provided by the metal traces and componentscomprising the MCM. There was no additional regulation beyond thatprovided by the Zener diode.

After providing this additional filtering, energy spectra, noise levels,and count rates from 57Co irradiation taken with the module 100described were well within the typical ranges for the same data takenwith standard modules 100 that receive VDET from an external supply.

These test results demonstrate that charge pump power supplies inaccordance with this invention are suitable for onboard generation ofsupply voltages for low-noise detector modules 100 for imaging systemswithout degrading the noise performance of such modules 100.

Because CPPS in accordance with this disclosure use a charge pumpingmethod of voltage generation, they are inherently efficient in energyuse.

In addition, these results also demonstrate that CPPS in accordance withthis invention and with the attributes of (d) low component counts, (e)minimal shielding, (f) minimal filtration and (g) minimal regulation cangenerate said supply voltages for low-noise detector modules 100 forimaging systems without degrading the noise performance of such modules100. Because of the small size of the module 100 of the practicalembodiment, the small size of the CPPS was essential. A plurality ofcomponents such as the scintillator arrays, the MCM and coolingcomponents occupy significant fractions of the volume of the module 100.

Each of the four attributes (d), (e), (f) and (g) above by itself lowersCPPS complexity and contributes to reduction of the size, weight, andcost of the CPPS and to increasing its reliability and manufacturingyield, thereby contribution to contributing to attainment of thecorresponding objectives for the module 100 and system.

The output voltage, VDET, of a CPPS per this invention may be adjustedthrough selection of a Zener diode 240 with an appropriate Zenervoltage, VZ, to optimize the performance of the associated module 100itself or with respect to other modules 100 in a multi-module system.

A number of embodiments of the invention have been described.Nevertheless, it will be understood that various modifications may bemade without departing from the spirit and scope of the invention.Accordingly, other embodiments are within the scope of the followingclaims. In addition, any modifications or substitutions that would beconsidered by one having ordinary skill in the art to be predictable,are intended to be encompassed within this disclosure.

For example, while the above describes the charge pump which operatesusing an inductor, a simple charge pump can simply operate using atransistor and capacitor. Moreover, while this describes an in-linezener diode for voltage regulation, and integrated circuit typeregulator or no regulator at all can be used. Also, while thisembodiment shows use of a FET, any transistor, or any switch for thatmatter can be used. The capacitor can be substituted by other comparablecharge storage mechanisms, and may in fact comprise a bank of capacitorsor the like.

All such modifications are intended to be encompassed within thefollowing claims:

1. A device and/or method substantially as shown and described.